Images taken with a digital camera are often blurred due to imperfect focus, camera motion, and in-camera optical and sensor characteristics. As a result, these images are often post-processed to increase sharpness. This accounts for the many commercially available digital image processing software applications that include image sharpening features. In addition, many digital cameras now include in-camera sharpening as part of a post-processing stage before the images are output.
Typically, these sharpening operations involve the use of some form of high-pass filter. Unfortunately, one of the major drawbacks associated with sharpening is its tendency to produce a halo effect (which is sometimes called ringing). The halo effect is characterized by bright regions of the image that are adjacent to dark regions becoming brighter than their surrounding pixels. In addition, dark regions of the image that are adjacent to bright regions get darker than their surrounding pixels.
U.S. Pat. No. 7,809,208, issued Oct. 5, 2010, describes an image sharpening technique with halo suppression. Generally, this technique reduces the haloing effect caused from image sharpening by restricting values to within local minimum and maximum intensities of the unsharpened image. Thus, if the sharpened value is below the local minimum, it is replaced with the local minimum. Similarly, the local maximum is taken, if the sharpened value exceeds the local maximum. This patent is incorporated herein by reference in its entirety.
As will be explained, the present invention provides a method for selective aperture sharpening and dark halo suppression by using chroma zones in CMOS imagers.